This invention relates to a motor control circuit and more particularly, to a motor control circuit which selectively controls the speed of a motor.
Motor control circuits are used to selectively measure and control the operating speed of a motor. Typically, these circuits utilize a rotary encoder which measures the current speed of the motor and a phase-locked loop type of circuit which compares the measured speed to a desired speed value and which provides an error signal having a value which is substantially equal to the difference between the desired and measured speed values. The error signal is then communicated to a comparator which compares the error signal to a certain reference signal which is generated by a pulse width signal modulation circuit. Based upon the comparison between the reference signal and the error signal, the comparator selectively generates and transmits a motor speed control signal to an electrical power source. The electrical power source is typically coupled to the motor and communicates electrical voltage and current to the motor, effective to allow the motor to operate at a certain speed. The motor speed control signal, emanating from the comparator, is typically effective to modify the amount or level of electrical power (e.g. the duty cycle of the generated and communicated electrical current) which is communicated to the motor, thereby desirably changing the operating speed of the motor. While these prior motor control circuits do adequately control the speed of the motor they suffer from some drawbacks.
For example and without limitation, the pulse width modulation circuit, of these prior motor control circuits, typically utilizes a relatively high single frequency signal which is effective to cause the comparator to generate the previously described motor speed control signal. This single relatively high frequency signal therefore switches the power supply at a relatively high rate which causes the creation of significant switching type power losses due to the inability of the power supply, which is typically of the solid-state or semiconductor type, to instantaneously respond to the received motor speed control signal. This relatively high single frequency signal also creates relatively high levels or amounts of electromagnetic interference (EMI) and/or radio frequency interference (RFI); the amount or level of the produced EMI and RFI being directly proportional to the frequency of the produced signal. Further, this relatively high level or amount of EMI and RFI typically creates a xe2x80x9cspikexe2x80x9d of energy within a relatively narrow frequency range which undesirably interferes with the electronic assemblies and/or devices which are operatively contained within the device and/or assembly within which the motor resides, such as a vehicle.
There is therefore a need for a motor control circuit which desirably controls the operating speed of a motor while substantially reducing the amount of the switching type electrical power losses and the amount or level of EMI and RFI emissions.
It is a first object of the invention to provide a motor control circuit which overcomes at least some of the previously delineated drawbacks of prior motor control circuits.
It is a second object of the invention to provide a motor control circuit which overcomes at least some of the previously delineated drawbacks of prior motor control circuits and which selectively and automatically controls the operating speed of a motor.
According to a first aspect of the present invention, a circuit is provided for use in combination with an error signal production circuit of the type which produces an error signal indicative of the speed of a motor. The provided circuit includes a signal generator which generates pseudo-random signals; a wave shaping circuit which is coupled to the signal generator, which receives the pseudo-random signals, and which generates at least one signal based upon the received pseudo-random signals; a comparator which receives the at least one signal and the error signal, which compares the at least one signal with the error signal and, based upon the comparison, which generates a motor speed signal which is effective to control the speed of the motor.
According to a second aspect of the present invention a method for controlling the speed of a motor is provided. The method includes the steps of providing a desired speed; measuring the speed of the motor; creating an error value by comparing the desired speed with the measured speed; pseudo-randomly generating a plurality of signals; comparing the error value to one of the pseudo-randomly generated signals; and controlling the speed of the motor based upon the comparison between the error value and the one pseudo-randomly generated signal.
These and other features, aspects, and advantages of the invention will become apparent from the following detailed description of the preferred embodiment of the invention and by reference to the attached drawings.